Since your "magic number" is 50, the most likely reason is the following combination of configuration choices:

1. Your Tools -> Preferences -> Database - Advanced -> Sql Array Fetch Size is 50 (the default)
2. You choose to use a Java 1.7 rather than Java 1.6
3. You are not using the latest production release of SQL Developer

Either increase the Sql Array Fetch Size, downgrade to Java 1.6, or upgrade to the latest SQL Developer.